Unable to buy new aircraft designs (because they are too expensive, or simply take too long to get into service), and facing the prospect of unmanned aircraft (UAVs) displacing more and more manned ones, the American military is spending a growing chunk of its budgets on upgrading and refurbishing the combat aircraft they already have. This was not a deliberate, long term plan, but simply a reaction to shortages of new aircraft. A lot of the new electronics and weapons involved in these upgrades can also equip UAV designs still in development, so such efforts are a double win. more